This invention relates to cooling of a gas turbine engine. In particular, it relates to gas turbine engines having a gasifier turbine and a power turbine, the gasifier turbine driving the compressor section of the turbine while the power turbine is driven by exhaust gasses communicated from the gasifier turbine through a turbine casing section. The power turbine may be interconnected through a transmission with an electric generator or the like.
Gas turbine engines, as is well-known, operate most efficiently at high temperatures. However, the structure of the gas turbine engines and in fact all mechanical structures have maximum operating limits, at least as regards operating temperature. These maximum operating temperatures are prescribed by the materials utilized in the construction of an engine. To exceed these maximum operating temperatures may result in premature failure of the engine and in some cases disastrous disintegration of a high speed turbine. Nevertheless, it is desirable to operate a gas turbine at as high a possible temperature as may be obtained consistent with the cooling capacity of the engine and the maximum operating temperature of the materials utilized therein. Many schemes have been devised for the construction of various turbine vanes utilized in gas turbine engines. The most common scheme may be longitudinally oriented cooling passages through the turbine vanes coupled with cooling air communicated outwardly through these longitudinal bores. The problem associated with such a system is that the cooling air must be provided to a turbine wheel rotating at an exceptionally high speed. In addition to cooling turbine blades, the turbine nozzle vanes must similarly be cooled. Since the turbine nozzle vanes are fixed relative the engine, supplying cooling air to these vanes does not present the problem of providing the cooling air to the rotating turbine blades as described above. The nozzle vanes are generally fixed to the turbine casing both at the inner end and the outer end; therefore, cooling air provided to internal passages in the nozzle vanes must be bled off either to an annular plenum chamber surrounding the turbine or through bleed passages integrally formed with each nozzle vane.
Expansion of the turbine casing resulting from the increase in temperature at operating speed causes a unique problem. The turbine casing may expand so that efficiency in the turbine is lost due to expanding gasses slipping past the end of the turbine blade in an inefficient manner and causing unnecessary tip turbulence at the end of the turbine blade. Accordingly, it has been found appropriate to have a close clearance between the rotating turbine blade and the turbine shroud. Expansion of the turbine shroud must be matched with expansion of the turbine wheel and the turbine blade so that clearance between the turbine blade and turbine shroud remains substantially the same.
Another problem unique is gas turbine engines is keeping the flow of expanding gasses passing downstream toward the power turbine and outwardly toward the heat exchanger. In particular, the expanding gas leaves the combustor and passes through a plurality of fixed nozzle vanes wherein it is directed against the turbine blades mounted at the periphery of the turbine wheel. The gap between the turbine wheel and the mounting for the nozzle vanes presents a possible point for the hot expanding gas to enter the area between the turbine wheel and the turbine housing raising the temperature of the various bearing components therein to an unacceptable level. Therefore, it is desirable to provide a flow of cooling air passing outwardly through the gap between the turbine wheel and the turbine housing at a pressure greater than that of the hot expanding gasses. This cooling air passing outwardly at this point prevents entry of the hot expanding gasses into the area between the turbine wheel and the turbine housing. Provision is also made for cooling air to pass into the expanding gas stream downstream of the power turbine wheel.
In summary, cooling of gas turbine engines requires special design in several areas. First, the nozzle vanes, which operate at the highest temperature, being adjacent to the combustor, must be cooled. Secondly, the gasifier turbine blades, adjacent the nozzle, must also be cooled. The mounting structure of the nozzle vane and the shroud surrounding the gasifier turbine have also proved difficult to cool. Turbulence has been found to occur between the nozzle vane support structure and the turbine wheel and further hot exhaust gasses should not be allowed to pass into the area between the turbine wheel and the turbine housing. Similar problems occur downstream in the power turbine section; however, in view of the fact that the temperatures downstream of the gasifier turbine, specifically at the power turbine, are considerably lower than that at the gasifier turbine, it has not been found necessary in all cases to provide extensive cooling means such as longitudinal cooling passages in either the nozzle vanes in the power turbine or the power turbine blades themselves.